1. Field of the Invention
The present invention relates to a hydraulic machine arrangement, and in particular, a hydraulic machine arrangement including at least one hydraulic machine that may operate as a pump, a motor, or both.
2. Background Art
It is well known that hydraulic regenerative systems promise improved efficiency over electric regenerative systems incorporating a battery. Hydraulic regeneration involves using a pump connected in the vehicle drive train as a retarding device, and then storing the resulting high pressure fluid in an accumulator. On subsequent vehicle acceleration, the high pressure fluid from the accumulator is routed to a hydraulic motor and the stored energy is recovered in the form of mechanical work which drives the vehicle forward. A low pressure accumulator acts as a reservoir to make up for fluid volume variations within the high pressure accumulator, and also provides a charge pressure to the inlet side of the pump. Integral to a system such as this are hydraulic machines—i.e., hydraulic pumps, motors, or machines that can operate as both a pump and a motor as desired.
One method of modulating braking and driving forces in hydraulic regenerative systems is to incorporate a variable displacement hydraulic machine to operate in concert with an accumulator whose pressure is a function of its state of charge. Conventional variable displacement hydraulic machines may vary the piston strokes to achieve the desired power modulation. Such devices can be bulky, heavy and expensive. Moreover, they do not package easily in automotive passenger vehicles, especially in the front of a vehicle, where space is limited.
One way to overcome the limitations associated with conventional variable displacement hydraulic machines is to use a fixed displacement machine. Such a machine is generally smaller and lighter than its variable displacement counterpart, but it does not allow the power modulation required in most applications. One solution to this problem is to use a fixed displacement hydraulic machine in conjunction with a variable ratio hydraulic transformer to facilitate the desired power modulation. Systems utilizing transformers such as these are described in U.S. patent application Ser. No. 10/535,354, entitled “Hydraulic Regenerative Braking System for a Vehicle,” filed on 18 May 2005, which is hereby incorporated herein by reference.
As an alternative to a transformer, it may be desirable to have a system that included a relatively compact variable displacement hydraulic machine, thus eliminating the requirement of a separate variable ratio transformer. Variable displacement hydraulic machines are described in U.S. patent application Ser. No. 11/721,903, entitled “Hydraulic Regenerative Braking System and Method for a Vehicle,” filed on 15 Jun. 2007, and U.S. patent application Ser. No. 11/913,971, entitled “Hydraulic Regenerative Braking System for a Vehicle,” filed on 9 Nov. 2007, each of which is hereby incorporated herein by reference.